In existing hydraulic cylinders intended for telescopically displaceable crane arms and comprising two pistons which are movable coaxially relative to their cylinder, the end surface of the larger piston has a size which is equal to the inner end surface of the cylinder jacket. The return side of the piston, on the other hand, has a considerably smaller operative surface. When a hydraulic cylinder of this kind is used to increase or reduce the range of a telescopically movable crane boom, it is necessary that each piston moves at essentially the same speed, both in the outwards and the inwards directions. This requirement cannot be satisfied, as the above-mentioned large difference in piston surfaces at the front and rear sides of the piston means that if the fluid volumes are equal, the piston is imparted a considerably higher speed in the inwards direction than in the outwards direction. As a consequence of this increase of speed, a large volume of return oil must likewise be forced out of the cylinder in a limited period of time, which means that the flow velocity in the conduits increases to critical values, unless the conduits are dimensioned to accommodate the larger fluid quantities. However, it is expensive to have to oversize tubes and ipe conduits.
Upon rapid piston movements outwards, such as is the case when the crane boom is directed downwards and the load is heavy, one consequence of the large piston surface differential may be that there is not enough time to allow a sufficient quantity of oil to refill the piston return side in the cylinder, and therefore a vacuum may generate in this area, causing the piston, during its return movements, to strike back across the "void" in an uncontrolled manner. This constitutes an element of risk to the crane operator and anybody in the immediate vicinity of the crane. In addition, cavitation damages may occur.